Modern modulation schemes, such as .pi./4 DQPSK and MSK, impose deliberate phase and frequency changes on a carrier to transmit data. In .pi./4 DQPSK modulation, for example, two-bit data words, or symbols, sequentially produce one of four instantaneous phase shifts to modulate the carrier. A 00 data word causes a +45 degree phase change, a 01 causes a +135 degree change, a 10 causes a -45 degree change, and a 11 causes a -135 degree change. When demodulating, each switch in phase produces a symbol.
Because of these phase changes, it is often difficult to measure important parameters, such as the frequency of the carrier signal, that are useful in the design of modern cellular radio systems and classification of radio transmissions. The difficulty arises because the zero-crossings of the signal are shifted by the phase modulation. Depending on the modulating data, the number of zero-crossings in a given time is often different from what would be seen on an unmodulated carrier, thus making it difficult to accurately determine the unmodulated carrier frequency.
Existing systems for determining carrier frequency in phase-modulated signals suffer either from inaccuracy or relative complexity. Measuring devices such as frequency counters are relatively simple, but often produce inaccurate results. Because these devices measure average frequency by counting the number of cycles in a given time, they produce results that fluctuate randomly--and often significantly--around the true carrier frequency, if modulation is present.
Other relatively more complex systems exist that accurately measure carrier frequency and other signal-characterizing parameters by first determining phase deviation and then using the phase deviation to determine the other parameters. One such system is disclosed in an article by Raymond A. Birgenheier entitled, "Measuring the Modulation Accuracy of .pi./4 DQPSK Signals for Digital Cellular Transmitters", appearing in the Hewlett-Packard Journal, April 1991, Vol. 42, No. 2. The system receives as input a .pi./4 DQPSK phase-modulated signal and digitizes its amplitude as a function of time using analog-to-digital conversion. There must be a minimum of several samples per cycle using this method. These uniformly spaced components are treated as the in-phase components of the signal. The signal is then passed through a Hilbert transformer to derive the quadrature components of the signal. Phase is obtained by taking the arctangent of the quadrature signal divided by the in-phase signal and magnitude is obtained by computing the square root of the sum of the in-phase signal and quadrature signal. Once both phase and magnitude are determined, a variety of other signal-characterizing parameters can be obtained.
Based on the foregoing, one skilled in the art would prefer a system that provides accurate information for characterizing phase-modulated signals, but involves a relatively less complex phase determination technique that requires sampling commensurate with the modulation bandwidth.